6. Dictionaries (associative arrays)

  1. Another data type of collection of values - dictionary
    • Lists, tuples, and strings use integer indexes to access the values, called elements, stored in them. Let's try the next example. 
      records = [89, 45, 23, 66, 3, -5]  # a list
records[2] = 20
print(records[2])
records = (89, 45, 23, 66, 3, -5)  # a tuple
print(records[2])
name = 'Williams'
print(name[5])
    • Dictionaries use {...}, and indexes in a dictionary are keys. Keys can be strings and integers. But usually string values. Obviously those keys should be all different in a dictionary so that a specific value can be accessed through its key. Let's try the next example. 
      key5 = 'country'
records = {'name': 'John', 'id': 345678, 'age':20, 10: 'Kamloops', key5: 'Canada'}  # {...}; key5 ???
records['age'] = 19
print(records['age'])
print(records['address'])  # ???
    • Let's try the next example that reads values from the user and saves them into a dictionary. 
      def readRecord():
    records = {}  # empty records
    records['name'] = input("Name: ")
    records['id'] = int(input("Id: "))
    records['age'] = int(input("Age: "))
    return records

records = readRecord()
print(records)
    • Once thing that dictionaries are different from lists is that they cannot be sliced like lists. This is because values are not listed with integer indexes.
    • Can we take out keys from a dictionary and put them into a list? How? Type conversion function. Try the next example. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
keys = list(records)
print(keys)
for k in keys:
    print(k)
    print(records[k])
    print(k + "-> " + str(records[k]))
  2. Methods for dictionaries
    • How to get all the keys - .keys(). Let's try the next example. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
keys = records.keys()
print(keys)
for k in records.keys():
    print(k + "-> " + str(records[k]))
    • How to get all the values - .values(). Let's try the next example. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
values = records.values()
print(values)
for v in records.values():
    print(v)
    • How to get all the key-value pairs - .items(). Let's try the next example. The return values from items() are tuples, not lists nor dictionaries. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
items = records.items()
print(items)
for it in records.items():
    print(it)
    • A method to get a value with a key - .get(). Let's try the next example. get(k, v) returns the value of the key k when the k exists. If the key k does not exist, v will be returned. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
print(records['name'] + '\' lives in ' + records.get('country', 'Not defined'))
records['country'] = input("Country: ")
print(records['name'] + '\' lives in ' + records.get('country', 'Not defined'))
    • A method to set a default value - .setdefault(). Let's try the next example. setdefault(k, v) sets the value v of the key k only when the k does not exist. If the key k does not exist, v will be returned. Otherwise, the existing value will be returned. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}
print(records['name'] + '\' lives in ' + records.setdefault('country', 'Canada'))
print(records['name'] + '\' lives in ' + records.setdefault('country', 'US'))  # ???
print(records['country'])  # ???
records['country'] = 'US'
print(records['country'])  # ???
  3. Operators for dictionaries - in and not in
    • How to check if a key exists - in and not in. Let's try the next example. 
      records = {'name': 'John', 'id': 345678, 'age':20}  # {...}

if 'name' in records:
    print('Name: ' + records['name'])
if 'country' not in records:
    records['country'] = input("Country: ")
    print(records)

if 'country' in records.keys():
    print(records['country'])
if 'Canada' not in records.values():
    print("Not Canada")
  4. Nested dictionaries and lists
    • Let's try the next example of a dictionary of lists. 
      students = {'John': [20, 'CS', 'TRU'], 'Caren': [19, 'MATH', 'TRU'], 'Snape': [58, 'Potion', 'Hogwarts']}
print(students['John'])
    • Let's try the next example of a list of dictionaries. 
      students = [{'name':'John', 'age':20, 'major':'CS', 'school':'TRU'},
    {'name':'Caren', 'age':19, 'major':'MATH', 'school':'TRU'}, 
    {'name':'Snape', 'age':58, 'major':'Potion', 'school':'Hogwarts'}]
print(students[0])
    • Which style do you prefer?
    • Let's try the next example of a list of lists - tic-tac-toe board. 
      def printBoard(board):
    for r in range(len(board)):
        for c in range(len(board)):
            print(board[r][c], end='')  # end='' gives no new line
            if c == 0 or c == 1:
                print('|', end='')
        print()  # new line
        if r != 2:
          print('-+-+-')

board = [[' ', 'O', ' '], ['X', ' ', 'X'], [' ', ' ', ' ']]  # a tic-tac-toe board
print(board[0])
print(board[1])
print(board[2])
print(board[0][1])
printBoard(board)
    • Let's try the next example of a list of lists - 3×3 puzzle board. 
      def printHorzLine(board):  # n×n board
    for c in range(len(board[0]) * 2 - 1 + 2):
        if c % 2 == 0:
            print('+', end='')
        else:
            print('-', end='')
    print()

def printBoard(board):
    printHorzLine(board)
    for r in range(len(board)):
        print('|', end='')
        for c in range(len(board)):
            print(board[r][c], end='')
            if c != len(board[r]) - 1:
                print('|', end='')
        print('|')
        printHorzLine(board)

board = [['1', '2', '3'], ['4', '5', '6'], ['7', '8', ' ']]  # a 3×3-Puzzle board
printBoard(board)
  5. Programming exercises
    1. Write a program that reads multiple student records, saves them in a variable, and prints the average gpa. Each student record should be a dictionary having the keys, 'name', and 'gpa'.
    2. Tic-tac-toe game
      • Data structure for tic-tac-toe board
        • Using a dictionary, ???
        • Using a list, ???
        • Using a list of lists, ???
        • Using a list of lists - For example, [[' ','O',' '],['X','X',' '],[' ',' ',' ']] 
            |O|
 -+-+-
 X|X| 
 -+-+-
  | |
      • Write a function that receives a tic-tac-toe board and draws the board.
      • Write a program that plays tic-tac-toe. (Hint. The user can enter the board row and column numbers instead of mouse clicking.) (Refer the programming exercises c) in the previous chapter.)
    3. 3×3 puzzle game
      • Data structure for 3×3 board
        • Using a dictionary, ???
        • Using a list, ???
        • Using a list of lists - For example, [[4,8,5],[1,2,0],[3,6,7]] 
           +-+-+-+
 |4|8|5|
 +-+-+-+
 |1|2| |
 +-+-+-+
 |3|6|7|
 +-+-+-+
        • The empty tile - The integer value, 0, in the board
      • Data structure for a position (or location) of a time
        • A tuple of row and column, where row and column start from 0
        • For example, (1, 2)
      • Write a function that makes a 3×3 board. The board should have randomly scattered numbers from 0 to 8. (Note that 0 will be used as the empty tile in the game.) (Refer the programming exercises d) in the previous chapter.)
      • Write a function that receives a 3×3 board, and prints it. 0 should be printed as a space, i.e., ' '. For example, 
         +-+-+-+
 |4|8|5|
 +-+-+-+
 |1|2| |
 +-+-+-+
 |3|6|7|
 +-+-+-+
      • Write a function that receives a tuple represening a position of row and column. The function checks if 0 <= row < 3 and 0 <= column < 3, and returns the result.
      • Write a function that receives a 3×3 board, and a tuple of row and column. It checks if (row,column) is a neighbor of the empty tile, and returns the result.
      • Write a function that receives a 3×3 board, and returns a tuple of the position of the empty tile.
      • Write a function that receives a 3×3 board, and two tuples of position0(=(row0,column0)) and position1=((row1,column1)). It swaps the two values in the board, which are pointed by position0 and position1.
      • Write a program that plays the 3×3 puzzle game, using the above functions. (Hint. The user can enter the board row and column numbers instead of mouse clicking.)
        Here is a solution.
  6. References